not to be intimidated by it noting he plans to take the subway to work this morning as he usually does. more on this threat, we're joined by phone to msnbc terror correspondent roger cresse. credible but unconfirmed. what do we make of that? >> it means there's probably a single source who passed the information that got to u.s. intelligence. there's an obligation when you have that type of credible data to run its ground. when i used to do the job, there were four questions. so you ask on a threat, is it credible, specific, imminent, can you corroborate it? in this case, you had yes to three of the four questions. that's why this is being taken as seriously as it is. >> we hear about reports of bridges and tunnels, perhaps the use of car or truck bombings. we hear about pakistan. you put all of the pieces together, can you trace it back to a source, roger?